In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. Ion implantation systems are often utilized to dope a workpiece, such as a semiconductor wafer, with ions from an ion beam, in order to either produce n- or p-type material doping, or to form passivation layers during fabrication of an integrated circuit. When used for doping semiconductor wafers, the ion implantation system injects a selected ion species into the workpiece to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic, or phosphorus, for example, results in an “n-type” extrinsic material wafer, whereas a “p-type” extrinsic material wafer often results from ions generated with source materials such as boron, gallium, or indium.
Typical ion implantation systems include an ion source for generating electrically charged ions from ionizable source materials. The generated ions are formed into a high speed beam with the help of a strong electric field and are directed along a predetermined beam path to an implantation end station. The ion implanter may include beam forming and shaping structures extending between the ion source and the end station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the ion beam passes en route to the end station. During operation, this passageway is typically evacuated in order to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with gas molecules.
It is common for the workpiece being implanted in the ion implantation to be a semiconductor wafer having a size much larger than the size of ion beam. In most ion implantation applications, the goal of the implantation is to deliver a precisely-controlled amount of a dopant uniformly over the entire area of the surface of the workpiece or wafer. In order to achieve the uniformity of doping utilizing an ion beam having a size significantly smaller than the workpiece area, a widely used technology is a so-called hybrid scan system, where a small-sized ion beam is swept or scanned back and forth rapidly in one direction, and the workpiece is mechanically moved along the orthogonal direction of the scanned ion beam.
One widely-used technology is serial implantation, where individual workpieces are implanted by the scanned ion beam. In order to maintain uniformity of the implantation, the ion beam current is often measured during the implantation process, wherein a beam sampling cup (e.g., a Faraday cup) is placed near an edge or reversing point of the scanned ion beam. The beam scan width is generally dictated by the position of the sampling cup(s), instead of the size of the workpiece, such that the ion beam is fully scanned past the sampling cup(s) with adequate over-scan to produce reliable measurements. Since the sampling cups are often at locations that are significantly far from the workpiece edges, the scan width on implanters utilizing such edge sampling cups are typically required to be far greater than the size of the workpiece being implanted. During times when the ion beam is not impacting the workpiece, the ion beam does not contribute to additional dosage on the workpiece, and ion beam utilization tends to suffer in such implanters.